1. Field of the Invention
The present invention relates to an image-taking optical system. More particularly, the present invention relates to a slim-type image-taking optical system suitable for, for example, a digital appliance equipped with an image capturing capability that captures an image of a subject with an image sensor.
2. Description of Related Art
In recent years, more and more cellular phones and personal digital assistants (PDAs) have been equipped with a built-in digital still camera or a built-in digital video unit for taking in images. There has been a growing demand for such digital appliances to be more downsized in view of portability but also to offer a higher performance in providing image information. To achieve a higher performance in providing image information, the number of pixels included in the image sensor which takes in images has been increased. Following this increase in the number of pixels included in the image sensor, downsizing of an element composing one pixel has also been in development. However, the overall size of the image sensor inevitably increases, resulting in an increase in the required image size and further a higher resolution required for the increased number of pixels. As a conventional image-taking optical system, a coaxial optical system called a straight-type optical system has been used. With this type, increasing the image size and the resolution involves an increase in the number of lenses used and an increase in the full length, which can be assumed to go against the downsizing of cellular phones and PDAs. Therefore, approaches to downsizing and slimming-down needs to be made by using not the straight type but a different type of an image-taking optical system.
As an image-taking optical system of a type different from the straight type, an optical system that employs a prism having a reflection surface is well-known. With this type, downsizing and slimming-down of the image-taking optical system is achieved by bending an optical path with the reflection surface. Methods of bending an optical path include repeating reflection within one prism, arranging a plurality of prisms so as to perform reflection a plurality of times, and the like. In addition, known also is an optical system that bends an optical path by using one surface both as a total reflection surface and a transmission surface (hereinafter the surface functioning for this total reflection and transmission is referred to as a TIR surface). One of such optical systems is an optical system that employs a TIR surface as a curved surface to achieve downsizing and slimming-down. For example, patent document 1 proposes an imaging optical system that employs two TIR surfaces for two prisms so as to elongate the optical path. Patent document 2 proposes an imaging optical system that employs two TIR surfaces for two prisms so as to reduce the length from the incidence surface to the image surface for slimming-down. As employed in these optical systems, the structure such that one curved surface functions both as a reflection surface and a transmission surface has the potential for achieving downsizing and slimming-down of an optical system.
[Patent Document 1] Japanese Patent Application Laid-open No. 2001-166209
[Patent Document 2] Japanese Patent Application Laid-open No. HI 1-271618
However, the optical structure described in the patent document 1 results in a longer full length from the entrance surface to the image surface, making it difficult to take full advantage of a TIR surface in reducing the full length. The optical structure described in patent document 2, in which only two prisms are provided, has a limitation in providing a higher resolution. That is, the use of a TIR surface alone or the use of only a prism having a TIR surface has a limitation in achieving a higher performance while keeping a system compact.